The present invention relates to a printing method and apparatus therefor and a printing system for printing an image on a printing medium by scanning with a printhead.
As a conventional image printing apparatus, an ink-jet printing apparatus (printer device) which performs printing (recording) by discharging ink on a printing medium such as a print sheet or the like is known.
The ink-jet printing apparatus is characterized by low-level noise because of the non-impact type and the capability of full-color image printing using plural colors of ink. In late years, such ink-jet printing apparatus is quickly diffused in the market.
FIG. 2 is a perspective view showing a general arrangement of a conventional ink-jet printing apparatus.
In FIG. 2, a rolled-type printing medium 5 is conveyed via conveying rollers 1 and 2 and held between a set of feeding rollers 3 and sent in a direction indicated by the arrow f in the figure, along with rotation of a sub-scanning motor 15 connected to the feeding roller 3. Guide rails 6 and 7 are arranged in parallel to move a carriage 8 traversingly over the printing medium 5, and along with the movement of the carriage 8, a printhead 9 scans in a lateral direction. The carriage 8 loads heads 9Y, 9M, 9C and 9Bk respectively corresponding to yellow, magenta, cyan and black, each having a plurality of ink discharge orifices. Ink tanks for the four colors of ink are arranged corresponding to the heads.
A printing operation, the printing medium 5 is intermittently conveyed in a unit of a printing width of the printhead 9. While the printing medium 5 is stopped, the printhead 9 scans in the direction of an arrow P, and in synchronization with the scanning process, an ink drop corresponding to a binarized image signal is discharged from each of orifices of each of the heads, performing printing operation.
In such ink-jet printing apparatus, characteristics of a printing medium are important. Particularly, the ink blurring characteristic of a printing medium largely affects image quality. As an index indicating the ink blurring characteristic of a printing medium, "blur rate" is known. The blur rate indicates a magnification of a diameter of an ink drop discharged from an ink-jet nozzle and blurred on a printing medium. The blur rate is obtained by the following equation:
blur rate=(dot diameter on a printing medium)/discharged ink drop diameter).
For instance, assuming that a discharged ink drop having a diameter of 30 .mu.m forms a dot having a diameter of 90 .mu.m on a printing medium. The blur rate of the given printing medium is 3.0. In a printing medium having a low blur rate, an image printed by discharging ink drops on the printing medium has a high image density (dark), thus it is difficult to obtain a realistic high-quality image by using the printing medium having a low blur rate.
On the contrary, in a printing medium having a high blur rate, a printed image on the printing medium has a low image density (light), but has the following problem. In the serial scanning type ink-jet printing apparatus as shown in FIG. 2, the printhead 9 having plural ink discharge orifices in parallel scans in a direction of the arrow A as shown in FIGS. 3A and 3B, thereby printing an image having a width d in the sequence of (1), (2) and (3). The width d is determined by the number of ink discharge orifices and the density of orifices of the head 9 (printing density). For instance, in a case of a printhead having 256 discharge orifices and a printing density of 400 dots/inch (dpi), the width is obtained by: EQU 256.times.25.4/400=16.256 (mm).
In this case, if the discharged ink amount is small, a width of the printed image almost equal to the printing width d is obtained since ink absorption of the printing medium is sufficient. Thus, if the relative position of the printhead 9 is moved on the printing medium in the direction B for the width d and the printhead 9 scans in the direction A, the image connection created by each print scanning operation is not problematic as shown in FIG. 3A.
However, in a high density (dark) portion, that is, a portion where the amount of ink discharged on the printing medium is large, a printing medium having a high blur rate cannot sufficiently absorb the ink, resulting in ink blur in a vertical direction (direction B) and the printed image width is widened to (d+.DELTA.d) (FIG. 3B). At this stage, if the relative position of the printhead 9 is moved in the direction B for the width "d", the printed images are overlapped for the width ".DELTA.d", causing to form a high-density portion (black line) as shown in FIG. 3B. On the other hand, if the relative position of the printhead is moved in the direction B for the width (d+.DELTA.d), a low density (light) portion where the discharged ink amount is low would have a printing width d, thus a white line is formed between the printed images.
The blurred width .DELTA.d of the printed image in a high-density portion, is influenced by the blur rate of a printing medium and the ink amount discharged on the printing medium. The higher the blur rate is, and the larger the discharged ink amount is, the blurred width .DELTA.d becomes large. Therefore, in order to prevent from forming the aforementioned black line, it is necessary to use a printing medium having a low blur rate or reduce an ink amount in printing operation. However, in such case, an image density of an obtained image becomes low; as a result, a realistic high-quality image cannot be obtained.
In order to solve the foregoing problem, a multi-scanning method has been suggested to obviate the black line formed in a printed image. The printing method representing the characteristics of the multi-scanning method will be described with reference to FIG. 4.
Generally, in the multi-scanning method, nozzles of a printhead 401 are divided into three sections X, Y and Z whose ink discharge orifices are referred to as X-1 to X4, Y-1 to Y4 and Z-1 to Z-4 respectively. In the first scanning operation, only the Z section of the nozzles is used to perform printing operation in a portion Z' of a print sheet, thinning out print data to 1/3 of one-scanning data. Then the print sheet 402 is moved in the direction B for the width d, and a portion indicated by Y' is printed by the Y section of the nozzles of the head 401, thinning out print data to 1/3 of one-scanning data, thereby printing the dots thinned out by the previous printing operation in the portion Z'. Moreover, the print sheet 402 is further moved in the direction B for the width d, and a portion indicated by X' is printed by using the X section of the nozzles. Upon printing the portion X', the dots thinned out by the previous printing operation in the portions Y' and Z' are printed. As set forth above, as illustrated in FIG. 4, the printing operation is performed in such manner that the dots printed by the same nozzle are not printed successively in the direction A. Therefore, the multi-scanning method provides an advantage in that a disturbed image formed by irregular discharging of ink drops becomes non-obvious.
However, there is a difference in density and colors of a printed image between the case where the image is formed by the conventional normal singles-canning operation and the case where the image is printed by the above described multi-scanning method. One of the causes is in that, by performing multi-scanning, an ink amount discharged at one time on a printing medium is decreased, resulting in different ink blur. The other cause is in that a landing position of an ink drop on a printing medium is displaced. The problem due to the latter cause can be solved by improving the precision of paper feeding or the precision of head positioning control. However, in the case of high-density printing, positioning of the landing position needs a higher degree of accuracy. Also, the precision of paper feeding largely varies depending on a type of print sheets, thus it is difficult to realize precise positioning control.